mineral
Iolite (Cordierite)
Cordierite (Magnesium Aluminum Silicate), Mg2Al4Si5O18
AI-generated identification · may be incorrectHardness: 7-7.5. Color: Violet-blue, grayish-blue, or indigo. Luster: Vitreous. Crystal structure: Orthorhombic. Cleavage: Distinct in one direction. Pleochroism: Strong (appears different colors from different angles).
- Hardness
- 7-7
- Color
- Violet-blue, grayish-blue, or indigo
- Luster
- Vitreous
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Physical properties
Hardness: 7-7.5. Color: Violet-blue, grayish-blue, or indigo. Luster: Vitreous. Crystal structure: Orthorhombic. Cleavage: Distinct in one direction. Pleochroism: Strong (appears different colors from different angles).
Formation & geological history
Usually forms during the contact or regional metamorphism of argillaceous (clay-rich) rocks. It is commonly found in hornfels or gneisses.
Uses & applications
Used primarily as a gemstone (known as 'Water Sapphire'), in industrial ceramics due to its low thermal expansion, and historically for navigation.
Geological facts
Iolite is famously known as the 'Viking's Compass.' Because of its extreme pleochroism, Viking navigators used thin slices of iolite to locate the sun on overcast days by checking the light polarization.
Field identification & locations
Identify in the field by its blue-to-violet color and the 'color shift' seen when rotating the stone. Commonly found in India, Sri Lanka, Brazil, and parts of Canada.
